Cross-channel Flow Research Articles

Three-dimensional structures and symmetry breaking in viscoelastic cross-channel flow.

Using holographic particle tracking, we report the three-dimensional flow structure organizing the viscoelastic instability in cross-channel flow. Beyond a critical Wi, the advective core flow undergoes an out-of-plane instability marked by the emergence of tertiary flow, resembling that of the toroidal vortices in Taylor-Couette geometry. The out-of-plane flow component distorts the separatrix between the impinging inflow streams, triggering symmetry breaking normal to the extension plane. As extensional rate increases, progressively higher order modes of the separatrix are observed, akin to Euler buckling of a rigid column. The disturbances propagate upstream via stress fluctuations despite viscous dissipation. These complex flow structures may be generic to elastic turbulence in mixed flows.

Intratidal and Subtidal Circulation in a Tropical Estuary during Wet Season: The Maroni, French Guiana

Observations of water level, current velocity, river discharge, wind and salinity were collected in the Maroni estuary, on the border of French Guiana and Suriname during the wet season of 2018 to explore subtidal circulation patterns. Measurements are complimented by the application of analytical models with an aim to diagnose forcing mechanisms responsible for producing subtidal flows during the day of data collection and to extrapolate these findings to other time periods with variable wind and river forcing. Subtidal along-channel flows were found to be dominated by river discharge, with seaward directed velocities found throughout the channel section reaching 40 cm s − 1 . This pattern was altered with strong southwesterly winds, which produced and inverse gravitational circulation pattern despite the elevated river discharge. Secondary, or cross-channel flows, displayed a three-layer vertical structure in the main channel due to a combination of channel curvature and tidal asymmetry in the lateral baroclinic pressure gradient. The pressure gradient was produced by a salinity intrusion front that only manifested in the channel during flood tide. This is the first comprehensive study of tidal and subtidal flow dynamics in the Maroni estuary.

Application of Network Analysis to Flow Systems with Alternating Wave Channels: Part A (Pressure Flows).

Wave-dispersion screws have been used industrially in many types of extrusion processes, injection molding, and blow molding. These high-performance screws are constructed by replacing the metering section of a conventional screw with a melt-conveying zone consisting of two or more parallel flow channels that oscillate periodically in-depth over multiple cycles. With the barrier flight between the screw channels being selectively undercut, the molten resin is strategically forced to flow across the secondary flight, assuring repeated cross-channel mixing of the polymer melt. Despite the industrial relevance, very few scientific studies have investigated the flow in wave-dispersion sections in detail. As a result, current screw designs are often based on traditional trial-and-error procedures rather than on the principles of extrusion theory. This study, which was split into two parts, was carried out to systematically address this issue. The research reported here (Part A) was designed to reduce the complexity of the problem, exclusively analyzing the pressure-induced flows of polymer melts in wave sections. Ignoring the influence of the screw rotation on the conveying characteristics of the wave section, the results could be clearly assigned to the governing type of flow mechanism, thereby providing a better understanding of the underlying physics. Experimental studies were performed on a novel extrusion die equipped with a dual wave-channel system with alternating channel depth profiles. A seminumerical modeling approach based on network theory is proposed that locally describes the downchannel and cross-channel flows along the wave channels and accurately predicts the pressure distributions in the flow domain. The solutions of our seminumerical approach were, moreover, compared to the results of three-dimensional non-Newtonian CFD simulations. The results of this study will be extended to real screw designs in Part B, which will include the influence of the screw rotation in the flow analysis.

K-Channel: A Multifunctional Architecture for Dynamically Reconfigurable Sample Processing in Droplet Microfluidics

By rapidly creating libraries of thousands of unique, miniaturized reactors, droplet microfluidics provides a powerful method for automating high-throughput chemical analysis. In order to engineer in-droplet assays, microfluidic devices must add reagents into droplets, remove fluid from droplets, and perform other necessary operations, each typically provided by a unique, specialized geometry. Unfortunately, modifying device performance or changing operations usually requires re-engineering the device among these specialized geometries, a time-consuming and costly process when optimizing in-droplet assays. To address this challenge in implementing droplet chemistry, we have developed the "K-channel," which couples a cross-channel flow to the segmented droplet flow to enable a range of operations on passing droplets. K-channels perform reagent injection (0-100% of droplet volume), fluid extraction (0-50% of droplet volume), and droplet splitting (1:1-1:5 daughter droplet ratio). Instead of modifying device dimensions or channel configuration, adjusting external conditions, such as applied pressure and electric field, selects the K-channel process and tunes its magnitude. Finally, interfacing a device-embedded magnet allows selective capture of 96% of droplet-encapsulated superparamagnetic beads during 1:1 droplet splitting events at ∼400 Hz. Addition of a second K-channel for injection (after the droplet splitting K-channel) enables integrated washing of magnetic beads within rapidly moving droplets. Ultimately, the K-channel provides an exciting opportunity to perform many useful droplet operations across a range of magnitudes without requiring architectural modifications. Therefore, we envision the K-channel as a versatile, easy to use microfluidic component enabling diverse, in-droplet (bio)chemical manipulations.

Twenty first century cooling solution: Microchannel heat sinks

Due to rapid evolution in a wide range of technologies in twentieth century, heat dissipation requirement has increased very rapidly especially from compact systems. There is an urgent need for high-performance heat sinks to ensure the integrity and long life of these petite systems. Use of forced convection cooling has been limited by the requirement of the excessively high flow velocity and associated noise and vibration problems. Microchannel heat sink seems to be most reliable cooling technology due to its superior command over heat carrying capability. Understanding the flow boiling phenomena in microchannel heat sink experimentally and analytically has been topic of intense research in twenty first century. In this review paper, the experimental studies on flow visualization, pressure drop and heat transfer characteristics of microchannels presented by different researchers are summarized. Some different flow patterns observed in microchannel geometry such as bubble nucleation in thin film, periodic variation of flow pattern, flow circulation, bubble suppression and cross-channel flow are explained briefly. The influence of vapour quality, heat flux, mass flux and channel geometry on pressure drop and heat transfer characteristics of microchannel are reported. Different correlations reported for single and two phase heat transfer characteristics are compared. The comparative analysis showed that available single phase and two phase correlations are inconsistence and large variation is observed among these correlations for same channel geometry, fluid and operating condition. Different instabilities associated with microchannels are also briefly presented.

Instabilities in stagnation point flows of polymer solutions

A recently developed microfluidic device, the optimized shape cross-slot extensional rheometer or OSCER [S. J. Haward, M. S. N. Oliveira, M. A. Alves, and G. H. McKinley, “Optimized cross-slot flow geometry for microfluidic extensional rheometry,” Phys. Rev. Lett. 109, 128301 (2012)10.1103/PhysRevLett.109.128301], is used to investigate the stability of viscoelastic polymer solutions in an idealized planar stagnation point flow. Aqueous polymer solutions, consisting of poly(ethylene oxide) and of hyaluronic acid with various molecular weights and concentrations, are formulated in order to provide fluids with a wide range of rheological properties. Semi-dilute solutions of high molecular weight polymers provide highly viscoelastic fluids with long relaxation times, which achieve a high Weissenberg number (Wi) at flow rates for which the Reynolds number (Re) remains low; hence the elasticity number El = Wi/Re is high. Lower concentration solutions of moderate molecular weight polymers provide only weakly viscoelastic fluids in which inertia remains important and El is relatively low. Flow birefringence observations are used to visualize the nature of flow instabilities in the fluids as the volumetric flow rate through the OSCER device is steadily incremented. At low Wi and Re, all of the fluids display a steady, symmetric, and uniform “birefringent strand” of highly oriented polymer molecules aligned along the outflowing symmetry axis of the test geometry, indicating the stability of the flow field under such conditions. In fluids of El > 1, we observe steady elastic flow asymmetries beyond a critical Weissenberg number,Wicrit, that are similar in character to those already reported in standard cross-slot geometries [e.g., P. E. Arratia, C. C. Thomas, J. Diorio, and J. P. Gollub, “Elastic instabilities of polymer solutions in cross-channel flow,” Phys. Rev. Lett. 96, 144502 (2006)10.1103/PhysRevLett.96.144502]. However, in fluids with El < 1 we observe a sequence of time-dependent inertio-elastic instabilities beyond a critical Reynolds number, ${\mathop{\rm Re}\nolimits} _{crit} $ Re crit, characterized by high frequency spatiotemporal oscillations of the birefringent strand. By plotting the critical limits of stability for the various fluids in the Wi-Re operating space, we are able to construct a stability diagram delineating the distinct steady symmetric, steady asymmetric and inertio-elastic flow regimes in this idealized planar elongational flow device.

Simulation of a Side Discharge into a Cross Channel Flow on a Non-Uniform Rectangular Grid

Simulation of a Side Discharge into a Cross Channel Flow on a Non-Uniform Rectangular Grid

Creation and tidal advection of a cold salinity front in Storfjorden: 1. Polynya dynamics

[1] Hydrographical measurements from the Storfjorden polynya document the presence of an abrupt front in near-freezing water dividing saline water recently created by a polynya event, from less saline water originating further south. This event occurred days before the survey with estimated heat flux ∼400 W m−2 over the polynya. Brine-enriched shelf water (BSW) is observed downslope toward deeper parts of Storfjorden, and BSW from earlier polynya events overflows the sill. Current measurements from a nearby sound, Freemansundet, document tidal currents exceeding 80 cm s−1 that displaced the front back and forth beneath the measurement site on fast ice ∼400 m from the polynya edge. Front displacement of ∼12 km is documented and mainly due to the M2 component superimposed on a mean residual current of 0.28 m s−1 into the sound induced by southerly wind during the survey. Complex topography imposes baroclinic tidal currents with strong vertical shear in the fast ice-covered sound, and with significant cross-channel flow. Supercooling events indicated in the hydrographical time series, and likely enhanced frazil ice production, are associated with double-diffusive turbulent mixing when the salinity front passes. In this way, these measurements indicate a novel ice production process along the edge of tidally induced latent heat polynyas where salinity fronts are generated. Turbulence increases (decreases) during flood (ebb) due to the destabilization (stabilization) of the water column when the salinity front passes the measurement site. Double-diffusive turbulent mixing related to tidal advection of salinity front below fast ice is pursued in a companion paper.

Suspended Sediment Transport in a Rock-Bound, Macrotidal Estuary: Han Estuary, Eastern Yellow Sea

ABSTRACT Lee, H.J.; Park, J.Y.; Lee, S.H.; Lee, J.M., and Kim, T.K., 2013. Suspended sediment transport in a rock-bound, macrotidal estuary: Han estuary, eastern Yellow Sea. The Yellow Sea is fringed with deltas and estuaries on the Chinese and Korean coasts, respectively. The Korean estuaries, rock bound on the ria coast, are represented by the Han estuary, the largest in the eastern Yellow Sea. To reveal the relationship between extensive tidal flats and major channels in the Han estuary, suspended sediment transport was observed from the water column down to the bottom boundary layer. Transecting channels and deploying benthic tripods were accomplished at a number of critical sites during different spring tides, mostly under the fair-weather conditions of dry seasons in 2006–08. The hydrodynamic measurements unraveled the role of macrotidal currents in estuarine suspended transport. According to lateral momentum analyses, cross-channel flows were governed by two forces: rotation and surface slope. The ...

Simulations of lateral mixing in cross-channel flow

Cross flow phenomena between connected sub-channels are studied by means of numerical simulations based on lattice-Boltzmann discretization. The cross (that is lateral) transfer is largely due to macroscopic instabilities developing at two shear layers. The characteristic size and advection velocity of the instabilities favorably compare with experimental results from the literature on a geometrically similar system. The strength of the cross flow strongly depends on the Reynolds number, with cross flow developing only for Reynolds numbers (based on macroscopic flow quantities) larger than 1360. Mass transfer between the sub-channels has been assessed by adding a passive scalar to the flow and solving its transport equation. As a result of the intimate connection of cross flow and lateral mass transfer, also the mass transfer coefficient is a pronounced function of Re.

Effect of bottom stress formulation on modelled flow and turbidity maxima in cross-sections of tide-dominated estuaries

A three-dimensional numerical model with a prognostic salinity field is used to investigate the effect of a partial slip bottom boundary condition on lateral flow and sediment distribution in a transect of a tidally dominated channel. The transect has a symmetrical Gaussian cross-channel bottom profile. For a deep, well-mixed, tidally dominated channel, partial slip decreases the relative importance of Coriolis deflection on the generation of cross-channel flow patterns. This has profound implications for the lateral distribution of residual salinity that drives the cross-channel residual circulation pattern. Transverse sediment transport, however, is always found to be governed by a balance between advection of residual sediment concentration by residual lateral flow on the one hand and cross-channel diffusion on the other hand. Hence, the changes in the cross-channel distribution of residual salinity modify the lateral sediment distribution. For no slip, a single turbidity maximum occurs. In contrast, partial slip gives a gradual transition to a symmetrical density distribution with a turbidity maximum near each bank. For a more shallow, partially mixed tidal channel that represents the James River, a single turbidity maximum at the left bank is found irrespective of the near-bed slip condition. In this case, semi-diurnal contributions to sediment distribution and lateral flow play an important role in cross-channel sediment transport. As vertical viscosity and diffusivity are increased, a second maximum at the right bank again exists for partial slip.

Flood/ebb tidal asymmetry in a shallow sandy estuary and the impact on net sand transport

Flood/ebb tidal dominance plays a pivotal role in estuarine sediment transport. The Dyfi Estuary, mid-Wales, UK, is used as a case study, together with an idealized trumpet-shaped estuary morphology, to illustrate key processes giving rise to flood/ebb dominance. Observations made in the main entrance channel to the Dyfi indicate that the system is presently ebb-dominant with regard to both the relatively long duration of the ebb tide and also the faster ebb velocities. Predictions made across the mouth of the estuary using the Telemac Modelling System suggest further that the net sand transport presently occurs out of the estuary. In contrast, due to the varying distribution of channels and flats within the estuary, the tidal flow in the upper estuary is flood-asymmetric and here the net transport is up-estuary. Flood versus ebb dominance is shown to be attributable to the relative extent of the channels and sandflats in relation to the mean water level. An attempt is made to develop a simple metric to determine present-day transport pathways within an estuary in response to its cross-channel flow asymmetries.

A numerical investigation of the effects of compression force on PEM fuel cell performance

In the present study we report on numerical investigations into the effects of compression on the performance of a unit cell. The focus of this study is how the transport properties of the gas diffusion layer (GDL) material, specifically porosity and permeability, affect numerical predictions of cell performance. Experimental data of porosity and permeability of uncompressed and compressed GDLs were obtained using a porometer, and used in numerical simulations. A 3D model with two parallel channels and an membrane electrode assembly (MEA) is constructed for the calculations. Three different configurations of transport properties were tested, i.e. uniform uncompressed GDL properties, uniform compressed GDL properties, and non-homogeneous GDL properties. It is found that the non-homogeneous case shows noticeable differences in predicted cell performance. For the non-homogenous case, simulations with a pressure difference between two cathode channels were carried out to gain insight into the effect of cross-channel flow on the overall prediction of cell performance. We found that the cross-channel flow changes local current density distribution primarily on the high-pressure channel. The present study demonstrates the importance of the proper use of transport properties for the compressed portion of the GDL.

Persistence of plankton in flowing water

Models of river plankton frequently suggest that these passively drifting communities are limited to downstream sections of larger rivers. I examine this hypothesis using a passive drift model for populations in advective environments, followed by a comparison of predicted and observed plankton populations in rivers. Under the scenario of continuous downstream drift, much of the plankton found in rivers is not predicted to occur. However, much of the observed plankton in rivers is explained through the addition of cross-channel flow heterogeneity to the model. Empirical data support the model and predict that many river plankton populations may be drifting downstream at less than 30% of the average rate of downstream flow. Plankton collections in the slower-moving edges of rivers demonstrate densities of up to 240% higher than those in adjacent midchannel flows (p = 0.009). These slow-moving areas are important habitat for river plankton and likely play a large role in planktonic food webs within rivers. These results may help explain why river productivity often decreases as a result of the loss of flow heterogeneity within river channels through human modification to landscapes.

Rayleigh Bénard convection: dynamics and structure in the physical space

The main objective of this article is part of a research program to link the dynamics of fluid flows with the structure and its transitions in the physical spaces. As a prototype of problem and to demonstrate the main ideas, we study the two-dimensional Rayleigh-Benard convection. The analysis is based on two recently developed nonlinear theories: geometric theory for incompressible flows [10] and the bifurcation and stability theory for nonlinear dynamical systems (both finite and infinite dimensional) [9]. We have shown in [8] that the Rayleigh-Benard problem bifurcates from the basic state to an attractor A_R when the Rayleigh number R crosses the first critical Rayleigh number R_c for all physically sound boundary conditions, regardless of the multiplicity of the eigenvalue R_c for the linear problem. In this article, in addition to a classification of the bifurcated attractor A_R, the structure and its transitions of the solutions in the physical space is classified, leading to the existence and stability of two different flows structures: pure rolls and rolls separated by a cross the channel flow. It appears that the structure with rolls separated by a cross channel flow has not been carefully examined although it has been observed in other physical contexts such as the Branstator-Kushnir waves in the atmospheric dynamics [1,7].

Hybrid Approach to High-Frequency Microfluidic Mixing

We report the experimental verification of the predicted chaotic mixing characteristics for a polydimethylsioxane microfluidic chip, based on the mechanism of multistage cross-channel flows. While chaotic mixing can be achieved within short passage distances, there is an optimal side channel flow pulsation frequency beyond which the mixing becomes ineffective. Based on the physical understanding of a Poincaré section analysis, we propose the installation of passive flow baffles in the main microfluidic channel to facilitate high-frequency mixing. The combined hybrid approach enables chaotic mixing at enhanced frequency and reduced passage distance in two-dimensional flows.

Elastic Instabilities of Polymer Solutions in Cross-Channel Flow

When polymer molecules pass near the hyperbolic point of a microchannel cross flow, they are strongly stretched. As the strain rate is varied at low Reynolds number (< 10(-2)), tracer and particle-tracking experiments show that molecular stretching produces two flow instabilities: one in which the velocity field becomes strongly asymmetric, and a second in which it fluctuates nonperiodically in time. The flow is strongly perturbed even far from the region of instability, and this phenomenon can be used to produce mixing.

Toolbox for the design of optimized microfluidic components

A computational "toolbox" for the a priori design of optimized microfluidic components is presented. These components consist of a microchannel under low-Reynolds number, pressure-driven flow, with an arrangement of grooves cut into the top and bottom to generate a tailored cross-channel flow. An advection map for each feature (i.e., groove of a particular shape and orientation) predicts the lateral transport of fluid within the channel due to that feature. We show that applying these maps in sequence generates an excellent representation of the outflow distribution for complex designs that combine these basic features. The effect of the complex three-dimensional flow field can therefore be predicted without solving the governing flow equations through the composite geometry, and the resulting distribution of fluids in the channel is used to evaluate how well a component performs a specified task. The generation and use of advection maps is described, and the toolbox is applied to determine optimal combinations of features for specified mixer sizes and mixing metrics.

Boundary Layer Forcing of a Semidiurnal, Cross-Channel Seiche

Abstract The outer Strait of Juan de Fuca is a stratified, tidal channel about 100 km long, 20 km wide, and 200 m deep. Tidal currents of O(0.5 m s−1) occur at both diurnal and semidiurnal periods and there is a pronounced spring–neap variation in the stratification due to changes in tidal mixing. Vertical isotherm excursions of O(50 m) have previously been observed along the northern side of the channel that appear to be phase locked to the tidal currents. Analyses of recent ADCP and thermistor chain data confirm the isotherm excursions and find that they are accompanied by distinctive baroclinic structures in the horizontal currents that persist across spring–neap cycles, and from year to year. The authors find that the phase of the semidiurnal signal does not vary along the channel, as would be expected of an along-channel internal tide. Instead, comparison of the semidiurnal measurements with two-dimensional analytical and numerical models indicate that much of the baroclinic structure can be explained as a cross-channel, internal seiche that is locally driven by reversing Ekman forcing in the bottom boundary layer. Near the bottom, the vertical motions are kinematic, resulting primarily from cross-channel flow on the side slopes. In the middepths, the seiche appears to explain part but not all of the vertical motions. The seiche appears to persist and to maintain a similar mode shape across large, O(2), changes in stratification. In Juan de Fuca, it is expected that these motions affect the overall salt balance of the system by enhancing mixing between the upper and lower layers of the estuary.

Cross Channel Flow Velocity and Motion Trajectory of Polymers in a Helical Channel

Abstract In reference to the comparison of polymer flow along the helical channel in the screw-based plasticating system in dependence from a kinematical system, which was presented in the previous publication [1], further comparative studies on these models in the range of cross channel flow and resultant flow were conducted. Studies were carried out for both previously described models that is the model with a rotating barrel and the model with a rotating screw. It was stated that polymer cross channel flow velocity and its distribution is different in both models and in both coordinate systems (rectangular coordinate system and helical coordinate system). In a rectangular and helical coordinate system the highest values of the drag (redrag) cross channel flow velocity are the same, the differences occur in the distribution of these velocities. The distribution of the pressure flow velocity is different in both coordinate systems. The conducted analysis of the resultant flow lets us state the differences in the motion trajectory of the polymer particle in both compared models. The spiral lead of the motion trajectory of the polymer particle is higher when the barrel rotates than in the model with a rotating screw. The cause of it lies in different barrel and screw peripheral speeds. Different senses of rotation of the barrel and the screw cause laevorotatory helical motion trajectory when the barrel rotates, and dextrorotatory when the screw rotates, whereas the behaviour of the polymer particle in the vicinity of the screw flight - change of direction by π/2 is the same.